Radiotherapy is often carried out by directing a high-energy beam of x-rays, typically at an energy level of several MeV, towards a tumour within a patient. This beam damages tissue that it passes through, tumour tissue more so than healthy tissue, so is collimated to reduce the amount of healthy tissue that is exposed to the beam. It is also directed towards the patient from a number of different directions, each passing through the tumour or other target point, so that any particular section of healthy tissue only receives a fraction of the total dose delivered during that treatment session (or “fraction”). This can be done by mounting the beam source on a gantry that is rotatable around the patient, thereby allowing it to be delivered from any chosen angle. Usually, the patient will be supported so that the tumour lies at the meeting-point of the rotation axis and the central axis of the beam, a location known as the “isocentre”.
This so-called MV beam can also be used for imaging the patient. However, the contrast between different tissue types at this energy level is poor, and the beam will also be collimated in order to limit the dosage to the patient. For this reason, a lower-energy beam is typically used for imaging purposes, having an energy of several keV at which the contrast between tissue types is more marked. So-called KV beams are often provided on radiotherapeutic apparatus for carrying out diagnostic imaging prior to and during radiotherapy.
It is known to image a patient during radiotherapy treatment using both the therapeutic (MV) beam and a diagnostic (kV) beam. This allows an operator to control and review the therapy in real time, confirming that the therapeutic beam is directed and collimated correctly with respect to the patient and that the patient is in the correct position.
However, this normally requires two separate imaging systems, one for the MV beam and one for the kV beam. Efforts to combine kV and MV imaging on the same detector have suffered from a perceived need to switch off the MV beam completely during kV imaging so that it does not spoil the kV image. Two detectors are therefore provided. Often, these are located on the rotatable gantry that supports the radiation head in positions that are 90° apart, allowing both to operate in parallel and largely independently, absent issues such as scattering. This imposes a significant material cost, however.